Is the 17mm TS-E really 17mm?

A non telephoto (or reverse telephoto) of a given focal length will -at infinity focus - form a virtual image exactly that focal length from the film plane. For a 17mm lens that would be 17mm from the film plane.

Conversely, Any focal length that doesn't form the image it's distance from the sensor plane at infinity is some form of telephoto.

If you play with large format cameras the direct relationship between focal length and film plane is more apparent. And those bad boys are covering a lot of film dimension.